This invention relates to apparatus for symmetrically truncating positive and negative values of two's complemented binary signals. The apparatus is generally applicable to systems which process two's complemented binary signals. However, it has particular utility in processing chrominance signals in digital TV receivers and will be described with reference to this environment.
Truncation of binary numbers involves discarding some of the least significant bits (LSB's) of the binary signal. For binary signals in two's complement format, simple truncation produces an asymmetry between positive truncated numbers and negative truncated numbers. Truncation of positive numbers produces numbers which tend toward zero while truncation of negative numbers produces numbers which tend toward more negative numbers. Specifically, if we chose to retain the (M-N) most significant bits (MSB's) of an M bit number having a decimal value .vertline.V.sub.10 .vertline., a positive value is truncated to the most positive integer whose value is less than or equal to V.sub.10 /2.sup.N. On the other hand, a negative value is truncated to the largest integer whose value is more negative than or equal to -V.sub.10 /2.sup.N. For example, truncating the binary equivalent of the decimal number .+-.13 by simply discarding the two LSB's produces the values of +3 and -4 for the positive and negative values respectively. Suppose that the .+-.13 values represented the peak values of a digital signal corresponding to a sinusoid centered about zero. It will readily be appreciated that simple truncation of a binary representation of such a sinusoid will introduce a DC shift into the truncated signal. In many applications, the DC shift is insignificant relative to the magnitude of the average peak signal levels. However, in other systems such asymmetry produces undesirable results and must be avoided.